1. Field of the Invention
This invention relates to an electroconductive interconnecting member and a fuel cell having the member.
2. Related Art Statement
There has recently been noticed a fuel cell, particularly a solid electrolyte-type fuel cell, as an electricity-generating device. The fuel cell has a high electricity-generating efficiency, can use various fuels (naphtha, natural gas, methanol, reformed coal gas, heavy oil and the like), is low in the public nuisance, and further is free from the influence by the installation scale upon its electricity-generating efficiency. Therefore, the fuel cell is very hopeful technic.
Particularly, the solid electrolyte-type fuel cell (SOFC) has a very high energy transformation efficiency and other merits. In the SOFC, a large number of fuel cell elements are connected in series, and hence an air electrode of an element in the foregoing stage is connected to a fuel electrode of an element in the next stage through an interconnector. This interconnector does not directly contribute to the electricity generation, and it is desirable to use an interconnector having a resistance as low as possible. For example, it is necessary for an interconnector to have an electroconductivity of not lower than about 1 S.multidot.cm.sup.-1 at 1,000.degree. C. and an electron conductivity of substantially 99-100% at 1,000.degree. C., which is an ordinary operating temperature of SOFC. Further, one of the surfaces of an interconnector is exposed to an oxidizing atmosphere, and the other surface is exposed to a reducing atmosphere. Therefore, the interconnector is required to be a chemically stable material and a dense material free from the leakage of fuel. Further, it is necessary that an interconnector is made of an inexpensive material, does not substantially change its composition under oxidizing atmosphere and reducing atmosphere, does not react with other fuel cell-constituting members at 1,000.degree. C., has a negligibly low ion conductivity, does not cause destructive phase conversion within the temperature range of 25.degree.-1,200.degree. C., is low in the volatilization of oxide component at the operation temperature, and further has a thermal expansion coefficient approximately equal to the thermal expansion coefficients of the solid electrolyte, supporting member and the like.
As the interconnector, there are used LaCrO.sub.3, La.sub.1-x Ca.sub.x CrO.sub.3, La.sub.1-x Sr.sub.x CrO.sub.3, LaCr.sub.1-x Mg.sub.x O.sub.3 and the like. However, lanthanum chromite, which is not doped, is not always highly useful. Because, the lanthanum chromite, which is not doped, has an electroconductivity near the lower limit value of its electroconductivity, has a thermal expansion coefficient, which is not matched with the thermal expansion coefficient of other members constituting the fuel cell together with the interconnector, and causes its phase conversion from orthorombic structure to rhombohedral structure at about 275.degree. C. For example, the use of LaCr.sub.0.97 Mg.sub.0.03 O.sub.3 results in a good electroconductivity, but LaCr.sub.0.97 Mg.sub.0.03 O.sub.3 is fairly lower in the thermal expansion coefficient at 25.degree.-1,000.degree. C. than zirconia stabilized with calcia, for example (ZrO.sub.2).sub.0.85 (CaO).sub.0.15, or a solid electrolyte, such as (ZrO.sub.2).sub.0.9 (Y.sub.2 O.sub.3).sub.0.1, which has a high ion conductivity at 1,000.degree. C.
For example, La.sub.0.84 Sr.sub.0.16 CrO.sub.3 has a good electroconductivity, but La.sub.0.84 Sr.sub.0.16 CrO.sub.3 is fairly large in the oxygen ion void factor in the case where it is exposed to a fuel atmosphere at a high operating temperature necessary for it to be used in a high-temperature fuel cell. LaCr.sub.0.72 Mg.sub.0.03 Al.sub.0.25 O.sub.3 is low in the electroconductivity due to the presence of aluminum ion in a relatively high concentration.
Therefore, there is demanded the development of an interconnector, which has an electroconductivity as high as possible and is well matched with solid electrolyte in the thermal expansion coefficient. Further, the interconnector is required to have a high denseness in view of the structure of SOFC. However, when the powder sintering is effected, LaCaO.sub.3, which is not doped, is difficult to be formed into a dense body even by a sintering at 1,600.degree. C. Therefore, the development of a material having a good sinterability at low temperature is desired. Further, it is eagerly demanded that an interconnector having more improved strength as a constituting member of fuel cell is developed to enhance the durability of fuel cell and to improve further the yield in the production of fuel cell.